I'm crazy too! In my own way, different than yours of course.
"I know that my "addict" cheating husband isn't the real person that he is when he's sober."
He's made choices. the real person? I'm sorry...who he is right now is real. It's who he chooses to be right now.
Might the relationship with this "fiancé" blow up and he comes back crying to you, promising to be good, clean up his messes?
Sure...but it will be because the life he's currently leading doesn't work out, so he will try to get back what he had.
His motivation will be purely selfish.
Who he is right now is as real as he ever was.
I feel for you because it seems you want to believe that there are two people inside him, and one is the good guy that loves you, and this other one is the bad guy who doesn't realize what he is doing.
He knows the choices he is making.
You're much better off working on that new life you are going to have when this divorce is final. Have you spent much time dreaming about it? Planning it? Looking forward to it? Look forward--not backward!